U.S. patent application number 10/873432 was filed with the patent office on 2005-12-22 for electronic device cooling assembly and method employing elastic support material holding a plurality of thermally conductive pins.
This patent application is currently assigned to International Business Machines Corporation. Invention is credited to Campbell, Levi A., Chu, Richard C., Ellsworth, Michael J. JR., Iyengar, Madhusudan K., Schmidt, Roger R., Simons, Robert E..
Application Number | 20050280993 10/873432 |
Document ID | / |
Family ID | 35480329 |
Filed Date | 2005-12-22 |
United States Patent
Application |
20050280993 |
Kind Code |
A1 |
Campbell, Levi A. ; et
al. |
December 22, 2005 |
Electronic device cooling assembly and method employing elastic
support material holding a plurality of thermally conductive
pins
Abstract
An electronic device cooling assembly and fabrication method are
provided which include a manifold with an orifice for injecting a
cooling liquid onto a surface to be cooled, and an elastic pin
support material with an opening aligned to the orifice of the
manifold. Multiple thermally conductive pins are mounted within the
support material, extending therefrom, and are sized to physically
contact the surface to be cooled. The support material has a
thickness and compliance which facilitates thermal interfacing of
the pins to the surface by allowing second ends thereof to move
vertically and tilt. The second end of each pin has a planar
surface which is normal to an axis of the pin, and the support
material facilitates the planar surfaces of the second pin ends
establishing planar contact with the surface to be cooled,
notwithstanding that the surface may be other than planar.
Inventors: |
Campbell, Levi A.; (New
Paltz, NY) ; Chu, Richard C.; (Hopewell Junction,
NY) ; Ellsworth, Michael J. JR.; (Lagrangeville,
NY) ; Iyengar, Madhusudan K.; (Rhinebeck, NY)
; Simons, Robert E.; (Poughkeepsie, NY) ; Schmidt,
Roger R.; (Poughkeepsie, NY) |
Correspondence
Address: |
HESLIN ROTHENBERG FARLEY & MESITI P.C.
5 COLUMBIA CIRCLE
ALBANY
NY
12203
US
|
Assignee: |
International Business Machines
Corporation
Armonk
NY
|
Family ID: |
35480329 |
Appl. No.: |
10/873432 |
Filed: |
June 22, 2004 |
Current U.S.
Class: |
361/699 ;
257/E23.093 |
Current CPC
Class: |
H01L 2924/0002 20130101;
H01L 2924/0002 20130101; H01L 23/4336 20130101; H01L 2924/00
20130101; H01L 2924/09701 20130101 |
Class at
Publication: |
361/699 |
International
Class: |
H05K 007/20 |
Claims
What is claimed is:
1. A cooling assembly for an electronic device, the cooling
assembly comprising: a manifold having at least one orifice therein
for injecting a cooling liquid onto a surface to be cooled, the
surface comprising one of a surface of the electronic device or a
surface in thermal contact with the electronic device; an elastic
pin support material having at least one opening aligned to the at
least one orifice of the manifold for allowing the cooling liquid
to pass therethrough; and a plurality of thermally conductive pins
mounted within the elastic pin support material and extending
therefrom, the plurality of thermally conductive pins being sized
to physically contact the surface to be cooled when the cooling
assembly is employed to cool the electronic device, wherein the
plurality of thermally conductive pins include first ends embedded
within the elastic pin support material, and wherein the elastic
pin support material has a thickness and compliance which
facilitates thermal interfacing of the plurality of thermally
conductive pins to the surface to be cooled by allowing second ends
thereof to move vertically and tilt, wherein the second end of each
thermally conductive pin comprises a planar surface which is normal
to an axis of the thermally conductive pin, and wherein when the
cooling assembly is employed to cool the electronic device, the
elastic pin support material facilitates the planar surfaces of the
second ends of the plurality of thermally conductive pins
establishing planar contact with the surface to be cooled,
notwithstanding that the surface may be other than planar.
2. The cooling assembly of claim 1, wherein the surface to be
cooled includes at least one of a concave surface region and convex
surface region, and wherein the elastic pin support material allows
thermally conductive pins of the plurality of thermally conductive
pins aligned over the at least one convex surface region or concave
surface region to move vertically and tilt so that the planar
second ends thereof are in planar contact with the surface.
3. The cooling assembly of claim 1, wherein each thermally
conductive pin of the plurality of thermally conductive pins
comprises a monolithic structure of uniform composition.
4. The cooling assembly of claim 3, wherein each thermally
conductive pin comprises either a metal, comprising at least one of
copper, chromium, gold, nickel, aluminum, silver, magnesium and
alloys thereof, or a non-metal, comprising at least one of silicon
carbide, diamond, graphite and aluminum nitride.
5. The cooling assembly of claim 1, wherein the elastic pin support
material is bonded to a surface of the manifold, wherein the
elastic pin support material is formed of a material comprising at
least one of polyether, polyester, filter foam, polyethylene,
silicone rubber and sponge, cellular urethane foam, silicone foam,
latex foam, open cell sponge, slow recovery foam, high density
urethane, conductive foam, and rubber based materials.
6. The cooling assembly of claim 1, wherein the electronic device
comprises at least one integrated circuit chip, and wherein the
surface to be cooled comprises one of a surface of the at least one
integrated circuit chip or a surface in thermal contact with the at
least one integrated circuit chip.
7. The cooling assembly of claim 1, wherein density of the
plurality of thermally conductive pins mounted within the elastic
pin support material varies from a center region of the elastic pin
support material to a periphery thereof.
8. The cooling assembly of claim 7, wherein density of the
plurality of thermally conductive pins increases from the center
region of the elastic pin support material to the periphery
thereof.
9. The cooling assembly of claim 1, wherein the plurality of
thermally conductive pins are mounted within the elastic pin
support material at an exposed surface thereof, and wherein the
exposed surface of the elastic pin support material is tapered from
a periphery of the elastic pin support material to a center region
thereof, wherein when the cooling assembly is employed to cool the
electronic device, with the plurality of thermally conductive pins
in physical contact with the surface to be cooled, the tapered
exposed surface of the elastic pin support material and the surface
to be cooled form a converging channel from a region aligned to the
center of the elastic pin support material to a region at the
periphery of the elastic pin support material.
10. The cooling assembly of claim 9, wherein density of the
plurality of thermally conductive pins increases from the center
region of the elastic pin support material to the periphery
thereof.
11. An electronic module comprising: a substrate; at least one
electronic device to be cooled disposed above the substrate; a
cooling assembly for the at least one electronic device, the
cooling assembly comprising: a manifold having at least one orifice
therein for injecting a cooling liquid onto a surface to be cooled,
the surface comprising one of a surface of the at least one
electronic device or a surface in thermal contact with the at least
one electronic device; an elastic pin support material, the elastic
pin support material having at least one opening aligned to the at
least one orifice of the manifold for allowing the cooling liquid
to pass therethrough; and a plurality of thermally conductive pins
mounted within the elastic pin support material and extending
therefrom, the plurality of thermally conductive pins physically
contacting the surface to be cooled, wherein the plurality of
thermally conductive pins include first ends embedded within the
elastic pin support material, and wherein the elastic pin support
material has a thickness and compliance which facilitates thermal
interfacing of the plurality of thermally conductive pins to the
surface to be cooled by allowing second ends thereof to move
vertically and tilt, wherein the second end of each thermally
conductive pin comprises a planar surface which is normal to an
axis of the thermally conductive pin, and wherein the elastic pin
support material facilitates the planar surfaces of the second ends
of the plurality of thermally conductive pins being in planar
contact with the surface to be cooled, notwithstanding that the
surface may be other than planar.
12. The electronic module of claim 11, wherein the surface to be
cooled includes at least one of a concave surface region and a
convex surface region, and wherein the elastic pin support material
allows thermally conductive pins of the plurality of thermally
conductive pins aligned over the at least one convex surface region
or concave surface region to move vertically and tilt so that the
planar second ends thereof are in planar contact with the surface
to be cooled.
13. The electronic module of claim 11, wherein each thermally
conductive pin of the plurality of thermally conductive pins
comprises a monolithic structure of uniform composition.
14. The electronic module of claim 13, wherein each thermally
conductive pin comprises either a metal, comprising at least one of
copper, chromium, gold, nickel, aluminum, silver, magnesium,
titanium and alloys thereof, or a non-metal, comprising at least
one of silicon carbide, diamond, graphite and aluminum nitride.
15. The electronic module of claim 11, wherein the elastic pin
support material is bonded to a surface of the manifold, and
wherein the elastic pin support material is formed of a material
comprising at least one of polyether, polyester, filter foam,
polyethylene, silicone rubber and sponge, cellular urethane foam,
silicone foam, latex foam, open cell sponge, slow recovery foam,
high density urethane, conductive foam, and rubber based
materials.
16. The electronic module of claim 11, wherein density of the
plurality of thermally conductive pins mounted within the elastic
pin support material varies from a center region of the elastic pin
support material to a periphery thereof.
17. The electronic module of claim 16, wherein density of the
plurality of thermally conductive materials increases from the
center region of the elastic pin support material to the periphery
thereof.
18. The electronic module of claim 11, wherein the plurality of
thermally conductive pins are mounted within the elastic pin
support material at an exposed surface thereof, and wherein the
exposed surface of the elastic pin support material is tapered from
a periphery of the elastic pin support material to a center region
thereof, wherein the tapered exposed surface of the elastic pin
support material and the surface to be cooled form a converging
channel from a region aligned to the center of the elastic pin
support material to a region at the periphery of the elastic pin
support material.
19. The electronic module of claim 18, wherein density of the
plurality of thermally conductive pins increases from the center of
the elastic pin support material to the periphery thereof.
20. The electronic module of claim 11, wherein the at least one
electronic device comprises at least one integrated circuit chip,
and wherein the electronic module further comprises a metal
barrier, said metal barrier having: a perimeter area, said barrier
perimeter area being sealably affixed to said substrate at its
periphery; an integrated circuit chip contact area, said integrated
circuit chip contact area being bonded and thermally coupled to an
upper surface of said at least one integrated circuit chip; and
wherein said surface to be cooled comprises a surface of the metal
barrier in the integrated circuit chip contact area.
21. A method of fabricating a cooling assembly for an electronic
device, the method comprising: providing a manifold having at least
one orifice for injecting a cooling liquid onto a surface to be
cooled, the surface comprising one of a surface of the electronic
device or a surface in thermal contact with the electronic device;
and providing an elastic pin support material having a plurality of
thermally conductive pins mounted therein and extending therefrom,
the elastic pin support material having at least one opening
aligned to the at least one orifice of the manifold for allowing
the cooling liquid to pass therethrough, the plurality of thermally
conductive pins being sized to physically contact the surface to be
cooled when the cooling assembly is employed to cool the electronic
device, wherein the elastic pin support material has a thickness
and compliance which facilitates thermal interfacing of the
plurality of thermally conductive pins to the surface to be cooled
by allowing second ends thereof to move vertically and tilt,
wherein the second end of each thermally conductive pin comprises a
planar surface which is normal to an axis of the thermally
conductive pin, and wherein the elastic pin support material
facilitates the planar surfaces of the second ends of the plurality
of thermally conductive pins establishing planar contact with the
surface to be cooled, notwithstanding that the surface may be other
than planar.
22. The method of claim 21, wherein each thermally conductive pin
of the plurality of thermally conductive pins comprises a
monolithic structure of uniform composition.
23. The method of claim 21, further comprising employing the
cooling assembly to cool the electronic device, the employing
including injecting cooling liquid onto the surface to be cooled
wherein the plurality of thermally conductive pins conduct heat
from the surface to be cooled and the cooling liquid removes heat
from the surface to be cooled and from the plurality of thermally
conductive pins by convection.
24. The method of claim 21, wherein the plurality of thermally
conductive pins are mounted within the elastic pin support material
at an exposed surface thereof, and wherein the exposed surface of
the elastic pin support material is tapered from a periphery of the
elastic pin support material to a center region thereof, wherein
when the plurality of thermally conductive pins are in physical
contact with the surface to be cooled, the tapered exposed surface
of the elastic pin support material and the surface to be cooled,
form a converging channel from a center region of the elastic pin
support material to a region at the periphery of the elastic pin
support material.
25. The method of claim 24, wherein density of the plurality of
thermally conductive pins increases from the center region of the
elastic pin support material to the periphery thereof.
26. The method of claim 21, wherein the electronic device comprises
part of an electronic module, the electronic module comprising a
substrate having a perimeter area, and the electronic device is
disposed above the substrate, and wherein the method further
comprises: sealably affixing a metal barrier to said substrate
perimeter area; bonding and thermally coupling said metal barrier
to the electronic device contact area; and wherein said surface to
be cooled comprises a surface of the metal barrier in the device
contact area.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application contains subject matter which is related to
the subject matter of the following application, which is assigned
to the same assignee as this application and which is hereby
incorporated herein by reference in its entirety:
[0002] "Electronic Device Substrate Assembly With Multilayer
Impermeable Barrier And Method Of Making", Chu et al., Ser. No.
10/197,661, filed Jul. 17, 2002.
FIELD OF THE INVENTION
[0003] The present invention relates to heat transfer mechanisms,
and more particularly, to heat transfer mechanisms and cooling
assemblies for removing heat generated by an electronic device, and
to methods of constructing such cooling assemblies. More
particularly, the present invention relates to cooling assemblies
with an elastic layer supporting a plurality of thermally
conductive pins capable of vertical and tilt movements for
facilitating physical, planar coupling of planar ends of the
plurality of thermally conductive pins with a surface to be
cooled.
BACKGROUND OF THE INVENTION
[0004] As is known, operating electronic devices produce heat. This
heat should be removed from the devices in order to maintain device
junction temperatures within desirable limits: failure to remove
the heat thus produced results in increased device temperatures,
potentially leading to thermal runaway conditions. Several trends
in the electronics industry have combined to increase the
importance of thermal management, including heat removal for
electronic devices, including technologies where thermal management
has traditionally been less of a concern, such as CMOS. In
particular, the need for faster and more densely packed circuits
has had a direct impact on the importance of thermal management.
First, power dissipation, and therefore heat production, increases
as device operating frequencies increase. Second, increased
operating frequencies may be possible at lower device junction
temperatures. Finally, as more and more devices are packed onto a
single chip, power density (Watts/cm.sup.2) increases, resulting in
the need to remove more power from a given size chip or module.
These trends have combined to create applications where it is no
longer desirable to remove heat from modem devices solely by
traditional air cooling methods, such as by using traditional air
cooled heat sinks. These trends are likely to continue, furthering
the need for alternatives to traditional air cooling methods.
[0005] One approach to avoiding the limitations of traditional air
cooling is to use a cooling fluid. As is known, different fluids
provide different cooling capabilities. In particular, fluids such
as refrigerants or other dielectric fluids (e.g., fluorocarbon
fluid) exhibit relatively poor thermal conductivity and specific
heat properties, when compared to fluids such as water or other
aqueous fluids. Dielectric fluids have an advantage, however, in
that they may be placed in direct physical contact with electronic
devices and interconnects without adverse affects such as corrosion
or electrical short circuits. For example, U.S. Pat. No. 6,052,284,
entitled "Printed Circuit Board with Electronic Devices Mounted
Thereon", describes an apparatus in which a dielectric fluid flows
over and around several operating electronic devices, thereby
removing heat from the devices. Similar approaches are disclosed in
U.S. Pat. No. 5,655,290, entitled "Method for Making a
Three-Dimensional Multichip Module" and U.S. Pat. No. 4,888,663,
entitled "Cooling System for Electronic Assembly".
[0006] Other cooling fluids, such as water or other aqueous fluids,
exhibit superior thermal conductivity and specific heat compared to
dielectric fluids. Water-based coolants, however, must be kept from
physical contact with electronic devices and interconnects, since
corrosion and electrical short circuit problems are likely to
result from such contact. Various methods have been disclosed for
using water-based coolants, while providing physical separation
between the coolant and the electronic devices. For example, U.S.
Pat. No. 4,531,146, entitled "Apparatus for Cooling High-Density
Integrated Circuit Packages", discloses the use of a conductive
foil barrier; U.S. Pat. No. 4,879,629, entitled "Liquid Cooled
Multi-chip Integrated Circuit Module Incorporating A Seamless
Compliant Member for Leakproof Operation", and IBM Technical
Disclosure Bulletin Vol. 20, No. 2, July 1977, entitled "Liquid
Cooled Module with Compliant Membrane", disclose the use of a
flexible barrier with thermal conduction enhancements (thermal
studs and heatsinks, respectively); and U.S. Pat. No. 4,381,032,
entitled "Apparatus for Cooling High-Density Integrated Circuit
Packages", and U.S. Pat. No. 5,294,830, entitled "Apparatus for
Indirect Impingement Cooling of Integrated Circuit Chips", disclose
the use of flexible barriers, where pistons are used to maintain
contact between the barrier and the devices to be cooled. The
above-incorporated, co-pending patent application entitled
"Electronic Device Substrate Assembly With Mulilayer Impermeable
Barrier and Method of Making", describes an electronic device
substrate assembly with a multilayer impermeable barrier which
provides high effective thermal conductivity between a device to be
cooled and a water-based coolant, while simultaneously maintaining
physical separation between the coolant and electronic devices and
minimizing mechanical stresses caused by mismatches in the thermal
coefficients of expansion of various materials within the device
assembly.
SUMMARY OF THE INVENTION
[0007] One method for implementing direct water-based cooling from
a heat transfer standpoint is to impinge one or more jets of
coolant liquid onto a surface thermally coupled to an integrated
circuit chip or electronic device to be cooled. In order to handle
very high heat fluxes (e.g., 200 W/cm.sup.2) with a reasonable
liquid flow rate, the jet orifices would need to be spaced close
together and be very small, for example, 0.25 mm diameter (or slot
width) or less. A concern with such small orifices is that they may
act to filter debris in the system, resulting in the orifices
becoming plugged. The orifices would need to be both plentiful and
small to achieve the high heat transfer coefficients needed to cool
a limited size, flat surface. One might compensate by creating
additional surface area in the chip (for example, by etching or
otherwise removing chip material), or adding extended surfaces to
the chip. Both require additional chip processing that is deemed
undesirable in terms of risking the loss of expensive functional
chips.
[0008] Therefore, a need exists for a practical cooling assembly
and method which allows for high heat transfer from a surface of an
electronic device to be cooled using a direct liquid coolant
impingment approach, without using structures likely to be
obstructed by debris in the system.
[0009] This need is met and additional advantages are provided
through a cooling assembly for an electronic device. The cooling
assembly includes a manifold having at least one orifice therein
for injecting a cooling liquid onto a surface to be cooled, and an
elastic pin support material. The surface to be cooled comprises
either a surface of the electronic device or a surface in thermal
contact with the electronic device. The elastic pin support
material has at least one opening aligned to the at least one
orifice of the manifold for allowing the cooling liquid to pass
therethrough. A plurality of thermally conductive pins are mounted
within the elastic pin support material and extend therefrom. The
plurality of thermally conductive pins are sized to physically
contact the surface to be cooled when the cooling assembly is
employed to cool the electronic device, and include first ends
embedded within the elastic pin support material. The elastic pin
support material has a thickness and compliance which facilitates
physical contact of the plurality of thermally conductive pins to
the surface to be cooled by allowing second ends thereof to move
vertically and tilt. The second end of each thermally conductive
pin comprises a planar surface which is normal to an axis of the
thermally conductive pin. When the cooling assembly is employed to
cool the electronic device, the elastic pin support material
facilitates the planar surfaces of the second ends of the pins
establishing planar contact with the surface to be cooled,
notwithstanding that the surface may be other than planar.
[0010] In another aspect, an electronic module is provided which
includes a substrate having at least one electronic device to be
cooled disposed thereon. A cooling assembly is provided for the at
least one electronic device. This cooling assembly includes a
manifold having at least one orifice therein for injecting a
cooling liquid onto a surface to be cooled and an elastic pin
support material. The surface to be cooled comprises either a
surface of the electronic device or a surface in thermal contact
with the electronic device. The elastic pin support material has an
opening aligned to the orifice of the manifold for allowing the
cooling liquid to pass therethrough. A plurality of thermally
conductive pins are mounted within the support material and extend
therefrom. The pins physically contact the surface to be cooled and
include first ends embedded within the support material. The
support material has a thickness and compliance which facilitates
thermal interfacing of the pins to the surface to be cooled by
allowing second ends thereof to move vertically and tilt. The
second end of each thermally conductive pin has a planar surface
which is normal to an axis of the thermally conductive pin. The
elastic pin support material facilitates the planar surfaces of the
second ends of the plurality of thermally conductive pins being in
planar contact with the surface to be cooled, notwithstanding that
the surface may be other than planar.
[0011] In a further aspect, a method of fabricating a cooling
assembly for an electronic device is provided. The method includes
providing a manifold having at least one orifice for injecting a
cooling liquid onto a surface to be cooled, the surface comprising
either a surface of the electronic device or a surface in thermal
contact with the electronic device; and providing an elastic pin
support material having a plurality of thermally conductive pins
mounted therein and extending therefrom, the support material
having an opening aligned to the orifice of the manifold for
allowing the cooling liquid to pass therethrough. The conductive
pins are sized to physically contact the surface to be cooled when
the cooling assembly is employed to cool the electronic device. The
support material has a thickness and compliance which facilitates
thermal interfacing of the pins to the surface to be cooled by
allowing second ends of the pins to move vertically and tilt. The
second end of each thermally conductive pin comprises a planar
surface which is normal to an axis of the thermally conductive pin.
The elastic pin support material facilitates the planar surfaces of
the second ends establishing planar contact with the surface to be
cooled, notwithstanding that the surface may be other than
planar.
[0012] Further, additional features and advantages are realized
through the techniques of the present invention. Other embodiments
and aspects of the invention are described in detail herein and are
considered a part of the claimed invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] The subject matter which is regarded as the invention is
particularly pointed out and distinctly claimed in the claims at
the conclusion of the specification. The foregoing and other
objects, features, and advantages of the invention are apparent
from the following detailed description taken in conjunction with
the accompanying drawings in which:
[0014] FIG. 1A depicts a top plan view of a passivated substrate
assembly, in accordance with an aspect of the present
invention;
[0015] FIG. 1B depicts a cross-sectional view of the passivated
substrate assembly shown in FIG. 1A, taken along line B-B;
[0016] FIG. 2 is a cross-sectional view of one embodiment of a
dual-mode conduction/convection cooling assembly, in accordance
with an aspect of the present invention;
[0017] FIG. 3A is a cross-sectional view of one embodiment of the
cooling assembly of FIG. 2 shown disposed over a surface of a
concave integrated circuit chip, in accordance with an aspect of
the present invention;
[0018] FIG. 3B is a cross-sectional elevational view of the
structures of FIG. 3A after the cooling assembly has been brought
into physical contact with the surface of the concave integrated
circuit chip so that the planar ends of the plurality of thermally
conductive pins are each in planar contact with the concave
surface, in accordance with an aspect of the present invention;
[0019] FIG. 4 depicts a partial cross-sectional view of an
alternate embodiment of an electronic module having a cooling
assembly with an elastic pin support material with a tapered
surface opposing the surface to be cooled, thereby defining a
converging liquid flow channel from near a center region of the
elastic pin support material radially outward, in accordance with
an aspect of the present invention; and
[0020] FIG. 5 is a plan view of another embodiment of pin placement
within an elastic pin support material of a cooling assembly,
wherein density of the plurality of thermally conductive pins
increases from a center portion of the support material outward
towards the periphery thereof, in accordance with an aspect of the
present invention.
BEST MODE FOR CARRYING OUT THE INVENTION
[0021] Generally stated, provided herein is an enhanced cooling
assembly and method which allows for high heat transfer from a
surface in thermal contact with an electronic device to be cooled
using a direct liquid coolant impinging approach. In one
embodiment, the cooling liquid is assumed to comprise a water-based
fluid, and the cooling assembly is employed in combination with a
passivated electronic substrate assembly. Other cooling liquids
could be employed, however.
[0022] The cooling assembly employs an elastic (i.e., low modulus
of elasticity) pin support material, such as an elastomeric
material or rubber-based material, to compliantly hold a plurality
of thermally conductive pins. The plurality of thermally conductive
pins are mounted within the support material and have second ends
extending therefrom. The pins are sized to physically contact a
surface to be cooled. The surface to be cooled can comprise either
a surface of an electronic device itself, or a surface thermally
coupled to the electronic device through which heat is extracted.
The thickness and compliance of the support material are fabricated
to a desired specification to allow the second ends of the pins to
simultaneously translate vertically and tilt. The second end of
each pin has a planar surface which is normal to an axis of the
pin. When in use, the support material facilitates the planar
surfaces of the second pin ends establishing planar contact with
the surface to be cooled, notwithstanding that the surface may be
other than a planar surface, for example, on a micron scale.
[0023] As a further enhancement, an exposed surface of the elastic
pin support material may be tapered from a periphery thereof to a
center region of the elastic pin support material. This tapering of
the exposed surface forms a converging channel between the support
material and the surface to be cooled from a center region of the
elastic pin support material to a region near the periphery
thereof. Additionally, or alternatively, density of the plurality
of thermally conductive pins can increase from a center portion of
the elastic pin support material to the periphery thereof to
further enhance conduction and convection of heat from the surface
to be cooled in regions near the periphery of the pin support
material. It is assumed herein that the elastic pin support
material is configured with a surface area approximating the
surface area of the surface to be cooled so that when the cooling
assembly is employed to cool the surface, the elastic pin support
material with the plurality of thermally conductive pins projecting
therefrom is aligned over the surface to be cooled.
[0024] Passivated Electronic Substrate Assembly
[0025] FIGS. 1A and 1B illustrate a passivated substrate assembly,
per one aspect of the present invention. As shown in FIGS. 1A and
1B, passivated substrate assembly 100 includes a substrate 110, and
one or more electronic device assemblies 130. Substrate 110
includes an upper surface 112, to which device assemblies 130 are
connected. Substrate upper surface 112 includes a perimeter area
114, surrounding the portion of substrate upper surface 112 that
contains device assemblies 130. Substrate 110 further includes a
plurality of external electrical connections, such as pins 116, as
illustrated in FIG. 1B. In one embodiment of the present invention,
substrate 110 is composed of ceramic or glass ceramic, however
other substrate materials as known in the art may be advantageously
used in conjunction with the teachings of the present
invention.
[0026] As shown in FIG. 1B, electronic device assembly 130 includes
an electronic device 132. Electronic device assembly 130 includes
an upper surface 134, which, in this embodiment, is the upper
surface of electronic device 132. Electronic device assembly 130
further includes electrical connections to substrate upper surface
112, such as solder ball connections 136 (also referred to herein
as controlled collapse chip connections, or C4s). Alternatively,
device 132 may be connected to substrate 112 through other means,
such as wires or leads, provided some portion of device assembly
upper surface 134 may be contacted by an electrically conductive
material.
[0027] FIG. 1B further illustrates the multilayer passivation
structure of the present invention. A first passivation layer 160
substantially occupies the volume adjacent to device assemblies
130, within the area bounded by substrate perimeter area 114, and
extending vertically from substrate upper surface 112, to
approximately the height of device assembly upper surface 134. As
illustrated in FIG. 1B, first passivation layer 160 need not occupy
the volume between devices 132 and substrate upper surface 112.
First passivation layer 160 may, however, optionally occupy the
volume between electronic device 132 and substrate upper surface
112, thereby substantially surrounding connections 136. A second
passivation layer 170 is located above the first passivation layer
and above the device assembly upper surfaces 134. Second
passivation layer 170 is shown in contact with device upper
surfaces 134, and thermally coupled thereto. Second passivation
layer 170 is in contact with substrate perimeter area 114, and
sealably affixed thereto.
[0028] The primary purpose of first passivation layer 160 is to
provide mechanical support for second passivation layer 170. Layer
160 should adhere to substrate upper surface 112, and to device
assembly 130. In particular, layer 160 should adhere to the sides
of device assembly 130, without significant defects, such as gaps,
cracks, voids, etc. The upper surface of layer 160 should be
relatively smooth and continuous, without significant defects.
Furthermore, the surface consisting of the upper surface of layer
160 and device upper surfaces(s) 134 should be relatively smooth
and continuous, without significant defects or discontinuities. In
this context, a defect or discontinuity is significant if it
impairs the function of layer 170, such as by creating a defect
(crack, gap, etc.) in layer 170 through which a cooling fluid might
pass. Whether a particular effect is significant, therefore, is
likely to depend on the properties of layer 170, such as the
materials selected, thickness, number of sublayers, etc. Layer 160
should be a poor electrical conductor, since layer 160 may be in
contact with connections 136, or electrical connections on
substrate upper surface 112. In preferred embodiments, layer 160 is
an electrical insulator. Layer 160 should further exhibit a low
dielectric constant, since layer 160 is likely to be in direct or
nearly direct contact with conductors on substrate surface 112, or
optionally with connections 136, any of which may carry high
frequency signals. The thermal transport properties of layer 160
are unimportant; as discussed below, thermal conduction is provided
by layer 170. Layer 160 may be composed of a single layer, or
multiple layers. Layer 160 may also be composed of a single
material, or multiple materials. In general, layer 160 may be
formed of a variety of materials, such as: silicones, acrylic
elastomers, thermoplastic elastomers, thermoplastic copolymers,
polysulfide polymers and polyurethanes, thermoplastic polyimides,
photosensitive polyimides, polyethersulfones and epoxies.
Alternatively, in embodiments of the present invention where layer
160 is formed of a plurality of materials, materials such as
engineering glass or silicon may be used to form a portion of layer
160. Other materials having the properties described herein may be
substituted by one of skill in the art, and are therefore within
the spirit and scope of the present invention.
[0029] Layer 160, or a portion thereof, may optionally serve a
second purpose, encapsulating connections 136 to reduce mechanical
fatigue and extend the useful life of connections 136. Mechanical
fatigue may be caused, for example, by subjecting assembly 100 to
temperature fluctuations, since assembly 100 is most likely
constructed of materials having varying thermal coefficients of
expansion. In embodiments of the present invention where mechanical
fatigue is a concern, an encapsulant such as, for example an epoxy
or a thermoplastic resin (such as polysulfone or polyetherimide) or
other suitable encapsulant, may be used to form layer 160, or a
portion thereof. Suitable encapsulants are described in, for
example, U.S. Pat. No. 5,656,862, Papathomas et al., entitled
"Solder Interconnection Structure", issued Aug. 12, 1997, U.S. Pat.
No. 5,668,059, Christie et al., entitled "Solder Interconnection
Structure and Process for Making", issued Sep. 16, 1997, and U.S.
Pat. No. 5,659,203, Call et al., entitled "Reworkable Polymer Chip
Encapsulant", issued Aug. 19, 1997, each of which is assigned to
the same assignee as the present application and each of which is
hereby incorporated herein by reference in its entirety. In such an
embodiment, the encapsulant should substantially occupy the volume
between electronic device 132 and substrate upper surface 112,
thereby substantially surrounding connections 136. The remaining
volume of layer 160 may be composed of the same encapsulating
material, or may optionally be composed of one or more of the
materials previously described (i.e., silicones, acrylic
elastomers, thermoplastic elastomers, etc.).
[0030] Second passivation layer 170 serves two purposes. First,
layer 170 provides an impermeable and corrosion resistant barrier,
capable of being in direct contact with a cooling fluid such as
water or other aqueous fluid. Second, layer 170 provides a high
effective thermal conductivity path between device assembly upper
surface 134 and a cooling fluid in contact with an upper surface of
layer 170. In addition, layer 170 and its deposition methods should
be compatible with layer 160. Each of these purposes, and the
structural and material implications thereof, are discussed in
detail in the above-incorporated U.S. patent application entitled
"Electronic Device Substrate Assembly with Multilayer Impermeable
Barrier and Method of Making".
[0031] Cooling Assembly with Elastic Pin Support
[0032] FIGS. 2-5 depict multiple embodiments of a cooling assembly
and fabrication method employing elastic support material holding a
plurality of thermally conductive pins, in accordance with an
aspect of the present invention. In these embodiments, the
thermally conductive pins are shown for simplicity directly
contacting an upper surface of an electronic device, such as an
integrated circuit chip. Alternatively, the plurality of thermally
conductive pins could thermally contact an upper surface of a
passivation layer, such as layer 170 of FIG. 1B. The upper surface
of the integrated circuit chips, and thus the upper surface of
second passivation layer 170 may include one or more concave
surface regions or convex surface regions, for example, resulting
from thermal cycling of the electronic module.
[0033] As used herein, "electronic device" comprises any heat
generating component of, for example, a computer system or other
electronic system requiring cooling. The term includes one or more
integrated circuit devices, semiconductor chips, and/or electronic
components, such as depicted in FIGS. 1A & 1B. The "surface to
be cooled" refers to a surface of the electronic device itself, or
to an exposed surface of a thermal cap, thermal spreader,
passivation layer, or other surface in thermal contact with the
electronic device, and through which heat generated by the
electronic device is to be extracted.
[0034] In one embodiment, the cooling assembly and cooling method
disclosed herein employ a direct water-based cooling approach with
characteristic flow dimensions that do not pose an orifice plugging
concern (e.g., employing orifices of 0.5 mm or greater), and which
can achieve a high rate of heat removal well in excess of 200
W/cm.sup.2. This is achieved by bringing an array of small
diameter, thermally conductive pins, for example, fabricated of
metal (such as copper), and on the order of 0.5 mm in diameter, in
good thermal, planar contact with the surface of the electronic
device to be cooled, and by causing the water-based cooling liquid
to flow around the pins after impinging upon the surface to be
cooled.
[0035] FIG. 2 depicts a partial cross-sectional view of a
dual-mode, conduction and convection cooling assembly in accordance
with an aspect of the present invention.
[0036] In FIG. 2, an electronic module 200 is partially shown which
includes a substrate 210, which may include conductive wiring (not
shown) on an upper surface thereof and/or embedded therein. One or
more electronic devices 220 are electrically connected to the
electrical wiring of substrate 210 via, for example, solder ball
connections 225. One embodiment of a cooling assembly, generally
denoted 230, is shown in physical contact with an upper surface of
electronic device 220. As noted above, the passivated electronic
substrate assembly of FIGS. 1A & 1B could be employed to
isolate the electronic devices from, for example, water-based
coolant. In that case, cooling assembly 230 would be in physical
contact with an upper surface of, for example, second passivation
layer 170.
[0037] Cooling assembly 230 includes a manifold 240 having one or
more orifices 245 therein for injecting cooling liquid onto the
surface of the electronic device to be cooled. Note that in FIGS.
2-4, only a portion of manifold 240, 440 comprising a rigid jet
plate is shown. Further details of the manifold are omitted as
being conventional and readily implementable by one skilled in the
art. In one embodiment, at least one orifice is provided disposed
(e.g., centrally) above each electronic device assembly to be
cooled. This allows the injection of cooling fluid directly onto
the surface to be cooled for the removal of heat from the
electronic device. Outlet orifices 247 are also provided in
manifold 240 for removing coolant after passing through the
assembly.
[0038] In accordance with an aspect of the present invention, an
elastic pin support material 250 is provided for holding an array
of thermally conductive pins 260. The array of thermally conductive
pins are in line or staggered, or of varying density as explained
further below. The pins can be fabricated of metal, for example,
comprising at least one of copper, chromium, gold, nickel,
aluminum, silver, magnesium, titanium and alloys thereof, or
non-metal, for example, silicon carbide, diamond, graphite and
aluminum nitride, and can comprise monolithic structures of uniform
composition. As shown, a first end of each pin of the plurality of
thermally conductive pins is embedded within the elastic pin
support material, while a second end of each pin, which physically
contacts the surface to be cooled, has a planar surface normal to
an axis of the pin to allow for good planar contact with the
surface, thereby facilitating good thermal interface between the
conductive pins and the surface to be cooled.
[0039] Elastic pin support material 250, which can be directly
attached to a surface of manifold 240, should be selected to have
the properties of a reasonable force versus deflection (i.e.,
compression) characteristic, while at the same time having a low
modulus of elasticity to allow each pin to simultaneously translate
and rotate in order for the free end of each pin, which is a planar
surface, to establish planar contact with the surface to be cooled.
The elastic pin support material, which may be bonded to a surface
of the manifold, can be formed of a material comprising at least
one of polyether, polyester, filter foam, polyethylene, silicone
rubber and sponge, cellular urethane foam, silicone foam, latex
foam, open cell sponge, slow recovery foam, high density urethane,
conductive foam, and rubber based materials. For example, a
cellular urethane foam (sponge rubber) such as offered by Pronat
Industries, Ltd. of Natanya, Israel could be employed, or a closed
cell foam or other rubber based product such as offered by S &
S Plastics, Inc. of Newark, N.J. could be employed. Those skilled
in the art will note that other elastomeric and rubber-based
compliant materials could be substituted for those noted above.
Again, the elastic pin support material preferably has a thickness
and compliance which facilitates thermal interfacing of the
conductive pins to the surface of the electronic device to be
cooled by allowing the free ends thereof to move vertically and
tilt simultaneously, thus allowing the planar surface of each
conductive pin to establish planar contact with the surface to be
cooled, notwithstanding that the surface to be cooled may be other
than planar, for example, due to curvature on the order of microns
of the supporting substrate and/or the electronic device(s)
disposed thereon.
[0040] FIGS. 3A & 3B depict an example of the compliance
desired in pin support material 250. In these figures, cooling
assembly 230 again includes a manifold 240, having at least one
orifice 245 therein and an elastic pin support material 250 bonded
to a surface of the manifold. Support material 250 includes an
opening 255 which is aligned to the orifice of the manifold 240. A
plurality of thermally conductive pins 260 are embedded at a first
end within the elastic pin support material 250. The free end of
each pin is a planar surface 265. In the example shown, the
electronic device 300 (e.g., integrated circuit chip) is shown to
have a concave curvature, which measurements have shown can be the
case in practice. In addition to accommodating the shape of the
surface to be cooled, by conforming to surface tilt, variations in
surface height are also simultaneously accommodated by vertical
translation of the pins. This can be important for multichip
modules where chips may be at different heights. In one embodiment,
the variations to be accommodated may be on the order of microns
(10 microns or less for a surface of an integrated circuit chip)
and approximately 2 mm or less for chip tilt.
[0041] Fabrication of the cooling assembly could comprise the
thermally conductive pins being inserted into molded holes in the
elastic pin support material, or the pins could be molded directly
into the material itself as part of the fabrication process, i.e.,
the thermally conductive pins could be present at the time that the
elastic pin support material is molded. The support material could
then be bonded, for example, glued or epoxied, to the manifold,
which in one embodiment may be made of plastic.
[0042] In operation, cooling of an electronic device is achieved by
cooling liquid, such as a water-based coolant, entering the
electronic module through an inlet plenum and passing through an
orifice in the manifold over a surface of an electronic device
assembly to be cooled. The orifice can be rectangular (i.e., a
slot) or circular in shape. The slot width or circular diameter
might be greater than 1 mm, which is significantly larger compared
to orifices of 0.25 mm or less described above. The cooling liquid
impinges on the surface to be cooled, resulting in a high heat
transfer in the vicinity of the orifice, due to the stagnation
region below the orifice that is set up about a center line of the
impinging fluid with respect to the jet orifice, as is understood
in the art. Within this stagnation region, a boundary layer
comprises a relatively thin layer atop the impingement surface,
meaning that greater heat transfer occurs in this region. However,
as the direction and momentum of the fluid changes to passing along
the flat impingement surface, the thickness of the boundary layer
increases, thereby reducing the heat transfer coefficient between
the fluid and the impingement surface. Thus, provided herein are a
plurality of thermally conductive pins which serve to conduct heat
from the surface of the electronic device to be cooled, and then
allow heat to be convected from the pins to the cooling liquid as
the liquid passes around the pins parallel along the surface to be
cooled.
[0043] FIG. 4 illustrates a further embodiment of a cooling
assembly, in accordance with an aspect of the present invention. In
this embodiment, a substrate 410 again supports one or more
electronic devices 420, which are electrically connected to wiring
on and/or within the substrate via solder ball connects 425. A
cooling assembly 430 includes a manifold 440, an elastic pin
support material 450 and a plurality of thermally conductive pins
460, similar to the cooling assembly 230 embodiment of FIG. 2.
Manifold 440 has a circular orifice 445 which provides an impinging
flow of cooling liquid onto a surface of the electronic device 420
(or, as noted above, onto a passivation layer in good thermal
contact with the upper surface of the electronic device). In this
embodiment, an exposed surface 470 of the elastic pin support
material 450 (i.e., the surface of the support material in opposing
relation to the surface of the electronic device assembly to be
cooled) is tapered from the periphery 490 thereof to a center
region 480 having the opening aligned to the orifice of the
manifold through which the cooling fluid is injected. This tapering
of the surface 470 of the elastic pin support material forms a
converging flow path (i.e., decreasing channel height) as the
cooling liquid moves radially outward from the center portion of
the cooling assembly, where in this embodiment the circular orifice
for injecting cooling fluid is located. Those skilled in the art
will understand that other converging flow path geometries are also
possible. As the cooling liquid flows radially outward from the
impingement or stagnation region, the velocity decreases linearly
with distance from the center point (i.e., the radius of the flow).
The converging flow path depicted in FIG. 4 is designed to
compensate for this decrease in velocity due to the radial
flow.
[0044] As shown in FIG. 5, and as a further alternative, pin
density of a cooling assembly 500 could vary from a center region
580 of elastic pin support material 550 to a periphery 590 thereof,
with pin 560 density shown increasing radially outward from center
region 580 towards periphery 590. Those skilled in the art will
note that other surface pin configurations are also possible. In
one embodiment, the shape of the exposed surface of the pin support
material can approximately mirror the surface configuration shape
of the electronic device to be cooled.
[0045] By way of more specific example, a cooling assembly in
accordance with an aspect of the present invention could employ
copper pin-fins embedded within the elastic pin support material
for removing heat from the back side of a 1 cm.sup.2 integrated
circuit chip (approximately 0.75 mm thick). Eighty pins of diameter
0.5 mm could be used, with the pins distributed on a 1 mm pitch. A
slot jet might be located centrally in the manifold, enabling
impingement of a water-based coolant at a temperature of, for
example, 32.degree. C. at a volumetric rate of 0.5 gpm. The
separation distance between the jet and the back side of the chip
might be fixed at 0.5 mm. Substantial thermal performance is
achieved in the range of 400-500 W/cm.sup.2, at a pressure drop of
approximately 8.5 psi, when the temperature difference between the
hottest part of the chip and the inlet coolant, is 60.degree. C.
For example, a pin-chip interface with water-based coolant as the
interstitial fluid and a bond line of 1 micron exhibits a unit
resistance of 1.6.degree. C.-mm.sup.2/W. By way of comparison,
solder or epoxy joints are not expected to be better than 5.degree.
C.-mm.sup.2/W.
[0046] Although preferred embodiments have been depicted and
described in detail herein, it will be apparent to those skilled in
the relevant art that various modifications, additions,
substitutions and the like can be made without departing from the
spirit of the invention and these are therefore considered to be
within the scope of the invention as defined in the following
claims.
* * * * *